The largest wildfire in the United States continued its destructive march through the Sierra Nevada on Tuesday, pushing further into Yosemite National Park and for the first time burning nearly to the edge of Hetch Hetchy Reservoir, the linchpin of the water supply for 2.6 million Bay Area residents from San Francisco to Silicon Valley.

Flames from the Rim Fire -- which reached 184,481 acres -- also forced the closure of several miles of Yosemite's famed Tioga Road on Tuesday afternoon. The fire advanced to within 300 yards of the Hetch Hetchy Reservoir's southern edges, and ash is accumulating on the water's surface, said Harlan Kelly Jr., general manager of the San Francisco Public Utilities Commission, which owns the reservoir.

In this Sunday, Aug. 25, 2013, photo, the Hetch Hetchy Reservoir is hazy with smoke from the Rim Fire fills the air in Yosemite National Park, Calif. (AP Photo/Jae C. Hong)
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Jae C. Hong
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"It is an emergency," Kelly said. "We are taking it seriously. We are very concerned."

Still, Kelly said that water quality for Bay Area residents has not deteriorated. Even if the agency has to temporarily shut off Hetch Hetchy water, he said, it has a six-month supply in Bay Area reservoirs and in loans from neighboring water districts.

Meanwhile Tuesday, more than 3,700 firefighters battled the blaze with helicopters, hoses, shovels and chain saws. The fire, which began on Aug. 17 in the Stanislaus National Forest near Groveland, was 20 percent contained and had burned 31 homes and 80 other structures.

"There's been good progress today, both in fire behavior and the lines we've created," said Tom Medema, a spokesman with the National Park Service. "We're waiting to see if the winds kick up. But humidities were up today, which helps to slow the fire."

Medema said full containment probably won't come for weeks.

San Francisco water officials are testing Hetch Hetchy water every hour at 20 different points between the reservoir and the Bay Area, and it remains as pure as it was before the fire, Kelly said.

That's because the pipe that delivers water from Hetch Hetchy starts 260 feet below the reservoir's surface. Any ash floating on the top could take weeks, he estimated, to descend to that level and build up near the pipe.

In a race against the fire, the agency has increased water releases from Hetch Hetchy to the maximum rate possible-- 302 million gallons a day. The water flows down 160 miles of gravity-fed pipes and tunnels to the Bay Area, where it is stored in Bay Area reservoirs like Crystal Springs in San Mateo County and San Antonio in Alameda County.

In addition to San Francisco, Hetch Hetchy serves 28 cities and water districts in four counties, including Palo Alto, Los Altos Hills, Santa Clara, Sunnyvale, Mountain View, Hayward and Fremont. San Jose receives only 3 percent of its water from Hetch Hetchy.

"Everybody is following it pretty closely," said Walt Wadlow, general manager of the Alameda County Water District, which uses Hetch Hetchy water for about 20 percent of what it supplies to 330,000 people in Fremont, Union City and Newark.

"We are concerned, but we have had thorough discussions with San Francisco, and we're pretty confident they are on top of it."

Hetch Hetchy water comes from melting Sierra snow, off granite peaks. It is treated with chloramine and ultra-violet light, but it is not filtered, like most drinking water, because of its pristine origins. If the fire and ash pollutes the water, San Francisco officials could filter it by routing it through their Sunol Valley water treatment plant.

"We hope to get up to the reservoir in the next few days and assess the situation," said Kelly of the SFPUC. "The sooner we get up there, the better."

The amount of ash and other particles in the drinking water, known as "turbidity," would have to increase 25-fold to exceed public health standards, and for officials to shut off Hetch Hetchy water, Kelly said. Even then, Bay Area reservoirs hold a three-month supply, and other water agencies, the Santa Clara Valley Water District and the East Bay Municipal Utility District, have agreed to provide enough to Hetch Hetchy customers for three additional months, if necessary, as part of longstanding partnerships.

The fire, spreading slowly east and northeast through forests of lodge pole pine and fir trees, is having a growing impact on Yosemite, just as Labor Day weekend is arriving.

By Tuesday it had burned 41,000 acres inside the park, nearly twice as much as the day before. Most of the charred area is in the remote, northwestern corner of the park.

Parks officials said they are seeing a reduction in visitors but stressed that the total area burned is only 5 percent of Yosemite's 761,000 acres.

"The fire is 15 miles away from Yosemite Valley, and the smoke is not affecting the valley," said Kari Cobb, a Yosemite spokeswoman. "Don't cancel your Labor Day plans."

Nevertheless, parks officials announced the unusual step of closing Tioga Road, the main east-west artery through the park, between Crane Flat and White Wolf. The closure will last at least through Labor Day, Cobb said, so that fire crews can thin brush growing alongside Tioga Road in case the fire, which is four miles north of the road, advances toward it.

"We're not cutting big trees, just branches and bushes," she said, "anything that would allow the fire to jump the road."

Yosemite's entrance along Highway 120 remains closed, but other entrances at Tioga Pass, Highway 140 and Highway 41 are open. The fire also is burning 7 miles from ancient sequoia trees at the Tuolumne and Merced groves, and firefighters are running sprinklers and taking other steps to keep the flames away.

Residents were ready to evacuate in the small foothills towns on the northern edge of the fire, along Highway 108, the Sonora Pass Highway, where the blaze still threatens 4,500 homes.

"It's smoky. It's real thick today," said Kailee Burleigh, a waitress at the Caffe Blossom in Twain Harte. "Business is good. There aren't many tourists, but firefighters have been coming in all day. The locals have been paying for their meals and drinks."